Some years ago, scientists had determined that fat releases leptin, and this hormone effects appetite and Basal Metabolic Rate (BMR). They had discovered a mechanism that defends against getting too fat, but it's a mechanism that gets broken when you eat too much junk food. No junk food was available on an evolutionary time scale.

But other studies of leptin levels give other results. It's not as simple as more fat meaning more leptin in the blood. It is generally accepted that this hormone secreted by fat cells tells us when to stop eating.

Recently, a scientific team concluded that in obese people leptin doesn't reach the brain to help regulate appetite. More than that, in obese people leptin affects the processes going into the brain, compromising learning and memory. The team claims that low levels of leptin could be associated to cognitive deficits in patients with disorders like type 2 diabetes. This condition could explain why patients who have diabetes also may have troubles with their long-term memory.

Last findings belong to a geriatric team at Saint Louis University School of Medicine. Testing an animal model, they found that mice receiving leptin were able to better navigate a maze. They also find that leptin works better in older mice than in young ones, even at a lower dose.